Conventionally, a compressor used for a vehicle air conditioning apparatus comprises an electromagnetic clutch that is disposed between the compressor and a driving source to transmit power.
The electromagnetic clutch selectively transmits power by an electromagnetic force or does not transmit power. For example, as shown in FIG. 5, the electromagnetic clutch is configured so that power is transmitted by attracting an armature 2 to a rotor 3 by means of the magnetic force of an electromagnetic coil 1 and combining the armature 2 and the rotor 3 together (for example, refer to Patent Document 1). In the configuration example shown in FIG. 5, the radial width of the armature 2 is divided into two parts, and the radial width of the rotor 3 is divided into three parts, whereby a contact surface (gap) 4 between the armature 2 and the rotor 3 is divided into four parts in the radial direction. In the explanation below, the contact surface 4 on the rotor 3 side is called an armature contact surface 4a, and the contact surface 4 on the armature 2 side is called a rotor contact surface 4b. 
Also, for example, as shown in FIG. 6A, the armature contact surface 4a of the rotor 3 is divided into three parts in the radial direction by two grooves 5 having a groove width a, whereby an inner peripheral ring 3a, a central ring 3b, and an outer peripheral ring 3c are formed. Each of the two grooves 5 is divided into a plurality of sections in the circumferential direction by bridges 6 connecting the inner peripheral ring 3a, the central ring 3b, and the outer peripheral ring 3c together.
As shown in FIG. 6B, the armature 2 is also divided into two parts, an inner peripheral part 2a and an outer peripheral part 2b, in the radial direction by a groove 7 having a groove width b. The groove 7 on the armature 2 side is also divided into a plurality of sections in the circumferential direction by bridges 8 connecting the inner peripheral part 2a and the outer peripheral part 2b together. The armature 2 in this case is formed by a plate produced by punching a plate-shaped raw material.